mesa: allow MESA_GL_VERSION_OVERRIDE to override the API type

Change the format to MAJOR.MINOR[FC]
For example: 2.1, 3.0FC, 3.1

The FC suffix indicates a forward compatible context, and
is only valid for versions >= 3.0.

Examples:
2.1:   GL Legacy/Compatibility context
3.0:   GL Legacy/Compatibility context
3.0FC: GL Core Profile context + Forward Compatible
3.1:   GL Core Profile context
3.1FC: GL Core Profile context + Forward Compatible

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/docs/envvars.html b/docs/envvars.html
index 49c9025..5d855c6 100644
--- a/docs/envvars.html
+++ b/docs/envvars.html
@@ -69,9 +69,25 @@
 to crash.
 This is a work-around for that.
 <li>MESA_GL_VERSION_OVERRIDE - changes the value returned by
-glGetString(GL_VERSION). Valid values are point-separated version numbers,
-such as "3.0". Mesa will not really implement all the features of the given
-version if it's higher than what's normally reported.
+glGetString(GL_VERSION) and possibly the GL API type.
+<ul>
+<li> The format should be MAJOR.MINOR[FC]
+<li> FC is an optional suffix that indicates a forward compatible context.
+This is only valid for versions &gt;= 3.0.
+<li> GL versions &lt; 3.0 are set to a compatibility (non-Core) profile
+<li> GL versions = 3.0, see below
+<li> GL versions &gt; 3.0 are set to a Core profile
+<li> Examples: 2.1, 3.0, 3.0FC, 3.1, 3.1FC
+<ul>
+<li> 2.1 - select a compatibility (non-Core) profile with GL version 2.1
+<li> 3.0 - select a compatibility (non-Core) profile with GL version 3.0
+<li> 3.0FC - select a Core+Forward Compatible profile with GL version 3.0
+<li> 3.1 - select a Core profile with GL version 3.1
+<li> 3.1FC - select a Core+Forward Compatible profile with GL version 3.1
+</ul>
+<li> Mesa may not really implement all the features of the given version.
+(for developers only)
+</ul>
 <li>MESA_GLSL_VERSION_OVERRIDE - changes the value returned by
 glGetString(GL_SHADING_LANGUAGE_VERSION). Valid values are integers, such as
 "130".  Mesa will not really implement all the features of the given language version